1. Field of the Invention
The present invention relates to an active matrix type liquid display system such as a display unit used in a personal computer.
2. Description of the Related Art
The conventional active matrix type crystal display system has used transparent electrodes formed on the interfaces between the liquid crystal and two substrates facing to each other as electrodes driving a liquid crystal layer. The cause is that the conventional active matrix type crystal display system has employed a twisted nematic display type in which liquid crystal is driven by directing the electric field applied to the liquid crystal in the direction approximately perpendicular to the interfaces.
On the other hand, an active matrix type liquid crystal display system has been proposed in Japanese Patent Application Laid-Open No. 56-91277 (1981) in which liquid crystal is driven by directing the electric field applied to the liquid crystal in the direction approximately parallel to the interfaces.
In the conventional technology employing the twisted nematic display type described above, it is required to form a transparent electrode such as indium tin oxide (ITO) typical. However, since the transparent electrode has an unevenness of approximately several ten nm on its surface, it becomes difficult to fabricate a fine active element such as thin film transistor (hereinafter referred to as “TFT”). Further, since the projected part of the transparent electrode is often apt to be detached and mixed into other parts such as electrode, the yield of the products has been largely decreased.
Furthermore, there have been a lot of problems on the aspect of image quality in the conventional technology. Especially, it has been difficult to obtain a halftone display since change in brightness is large when the view direction is changed.
Still further, in the active matrix type liquid crystal display system using switching transistor elements, it is required to provide a scanning electrode and a signal electrode for driving the switching transistor elements in addition to a pixel electrode for applying voltage or electric field to liquid crystal to modulate transmitted light or reflected light. The scanning electrode and the signal electrode fluctuate the voltage in the pixel electrode through the parasitic capacitance Cgs between the scanning electrode and the pixel electrode and the parasitic capacitance Cds between the signal electrode and the pixel electrode. In particular, since the voltage in the signal electrode is always fluctuated by image information, the voltage in the pixel electrode is fluctuated through the parasitic capacitance Cds between the signal electrode and the pixel electrode to decrease the contrast or to produce a bad image called as cross-talk.
In the type in which the electric field applied to the liquid crystal in the direction approximately parallel to the interfaces of the substrates, there has been problem that the parasitic capacitance Cds between the signal electrode and the pixel electrode becomes large comparing to the case of the twisted nematic display type, the cross-talk being large, the contrast being decreased depending on the image pattern. The cause is that since a common electrode in the type applying the electric field to the liquid crystal in the direction approximately parallel to the interfaces of the substrates is not formed over the whole surface of the substrate facing against the substrate having switching transistor elements different from in the case of the twisted nematic display type, the line of electric force from the signal electrode is not shielded and is terminated at the pixel electrode. Therefore, in the type applying the electric field to the liquid crystal in the direction approximately parallel to the interfaces of the substrates, the active matrix driving has a disadvantage in image quality.